The present invention relates to a current interruption type ignition system for an internal combustion engine.
There is known in the art a primary current interruption type ignition system for an internal combustion engine comprises an ignition coil including a primary coil portion and a secondary coil portion; an exciter inducing an AC voltage in synchronism with a rotation of the engine to be supplied to the primary coil portion of the ignition engine; a first semiconductor switch provided in parallel to the primary coil portion to conduct a current from the exciter therethrough; a second semiconductor switch provided on a controlled terminal of the first semiconductor switch so that the second semiconductor switch, upon conduction thereof, causes the first semiconductor switch to be turned off so as to abruptly conduct a primary current to the primary coil portion of the ignition coil; and a signal source generating an ignition signal at the ignition position of the engine.
There have been proposed two types of such ignition system. One of them further comprises a trigerring signal generator for the first semiconductor switch. In this system, a trigerring signal generated by the generator causes the first semiconductor switch to be turned on and the current from the exciter to flow through the first semiconductor switch, and then an ignition signal from the signal source causes the second semiconductor switch to be turned on and therefore the first semiconductor switch to be turned off. When the first semiconductor switch is interrupted, the exciter induces a high voltage resulting in a large primary current to the primary coil portion of the ignition coil. Thus, the secondary coil portion of the ignition coil has a high ignition voltage induced to cause an ignition plug to be sparked. In such ignition system, however, the trigerring signal generator for the second semiconductor switch have to be provided in addition to the exciter and the signal source, which causes a whole magneto to be complicated and large-sized, and its cost of production to be inevitably expensive.
Another type ignition system comprises an electrical resistor provided between the exciter and the controlled terminal of the first semiconductor switch and provided in series to the second semiconductor switch. In this system, the first semiconductor switch is turned on by the signal from the exciter through the electrical resistor and turned off by the second semiconductor switch which is turned on by the ignition signal from the signal source so that the primary current is supplied from the exciter to the primary coil portion of the ignition coil. With this system, the trigerring signal generator for the first semiconductor switch can be omitted, but has some drawbacks, which are described below. First, an ignition performance is degraded. More particularly, when the first semiconductor switch is turned off at the ignition position of the engine to generate a high voltage across a coil of the exciter, a current from the exciter passes through the resistor and the second semiconductor switch as well as through the primary coil portion of the ignition coil, which causes the primary current to be decreased, and therefore the ignition voltage to be lowered. Secondly, the first semiconductor switch tends to be broken if the connection of the ignition system is erroneous. More particularly, if the primary coil portion of the ignition coil is erroneously connected or disconnected, the first semiconductor switch has the high voltage applied from the exciter because the primary coil portion is removed out of the system. Thus, the first semiconductor switch is caused to be broken by the high voltage thereacross.